From Hat Yai Railway Station
you get get to anywhere in the south pretty easily either by bus,
minibus or choose to travel on by train, with a number of overnight rail
services heading all the way up to Bangkok.

Trains from Hat Yai run north to Bangkok, stopping at popular stations such as : Surat Thani (for boats to the islands of Koh Samui and Koh Phangan), Chumphon (for boats to Koh Tao) and Hua Hin.

If you want to travel to places like Phuket, Krabi and Phi Phi Islands on the west coast where there are no nearby train stations, you can either get off at Hat Yai Station and continue by minivan or bus, or continue on to Surat Thani where there are also minivans and buses over to the west coast.

Train from Singapore to Thailand via Butterworth Penang

Travelling this way, you
will have to stay the night either in Butterworth or catch the short
ferry ride over to George Town on Penang Island where you will have much
more accommodation choices and things to do and see.

Singapore to Thailand Via Kota Bahru

Travel by train from Singapore to Thailand on the scenic "Jungle Railway" up through the centre of peninsular Malaysia to either Wakaf Bahru (nearest station to Kota Bahru) or Pasir Mas, then catch a bus to the border crossing at Rantau Panjang.